Beam spill control for a synchrotron



Nov. 19,1968 F. H. G. LOTHROP 3,412,337

BEAM SPILL CONTROL FOR A SYNCHROTRON Filed Aug. 24. 1966 RF. POWER 46 SUPPLY AMPLITUDE CONTROL 43 FILTER DETECTOR v A 2 s ADDER TiT R o r h *2 SAMPLE r27 INTEGRATOR a HOLD IR IT 39 C CU r, 29 zERo VOLTAGE ti oETEcToRa 0 STOP SIGNAL 38 GENERATOR INVERTER INVERTER 31 5! ADDER ADDER f 33 32 INVERTER /52 IN VEN TOR.

FRED H. G. LOTHROP ATTORNEY.

3,412,337 BEAM SPILL CONTROL FOR A SYNCHROTRON Fred H. G. Lothrop, Lafayette, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Aug. 24, 1966, Ser. No. 574,819 8 Claims. (Cl. 328-228) ABSTRACT OF THE DISCLOSURE An electronic circuit for use with synchrotons for spilling a controllable quantity of the particle beam current at a controllable rate onto a target adjacent the beam orbit. The amplitude of the radio-frequency accelerating potential is reduced so that some of the particles lose phase stability and orbit into a target. The quantity .and rate of beam spill is stabilized by providing negative feedback from a detector measuring the quantity of current present in the circulating particle beam.

This invention relates generally to synchrotron type charged particle accelerators and more particularly to apparatus for effecting spilling or removal of particles from a synchrotron at a precise, predetermined rate. The invention described herein was made in the course of, or under, Contract W-7405 eng-48 with the United States Atomic Energy Commission.

In general, the most effective and convenient utilization of a charged particle beam requires that the beam be removed from the synchrotron and directed at externally disposed targets. The usual method of beam removal (spilling) requires that the orbital radius of the beam be either increased or decreased so that circulating particles strike a target suitably disposed adjacent the beam radius.

In many operations of a synchrotron, it is highly desirable to extract the beam at an accurately constant rate over a relatively long time interval. It is further desirable that control be provided whereby either a pre-selected fraction or a fixed quantity of the circulating beam can be spilled evenly over a selected time interval.

The orbital beam position of a sychotron is controlled by both the intensity of the magnetic guide field and the frequency of the signal at the accelerating electrode in accordance with the general operation of synchrotrons, as described, for instance, in the text Sourcebook on Atomic Energy by Glasstone, 1958, D. Van Nostrand Co., Inc., Princeton, N.J., on pages 262 to 268. On page 257 of the above-identified text, there is a description of the principle of phase stability wherein the charged particles in the beam are accelerated at a time after the peak potential of the high frequency signal occurs. Phase stability provides for automatic velocity control for'the various particles in the orbiting beam so that all the particles tend to circulate around the accelerator in a cluster and at the same velocity. Such result avoids losing many of the particles.

The present invention is an electronic control circuit through which accurate and readily adjustable control of the beam extraction process is obtained by amplitude modulating the signal at the accelerating electrode, such signal being decreased in amplitude to cause controlled phase instability for a selected fraction of the particles in the beam. The particles which fall out of phase are thereby caused to drift away from the normal path of the circulating beam and to strike a suitably disposed target.

It is an object of the present invention to provide a beam spiller for a charged particle accelerator in which the time rate of beam spill can be made nearly constant. It is another object of the present invention to pro- United States Patent "ice vide a beam spiller in which the quantity of beam spilled is adjustable with high accuracy.

It is another object of the present invention to provide a beam spiller in which control means are provided either for spilling a pre-selected fraction of the circulating beam particles or for spilling a fixed quantity of beam particles.

It is another object of the present invention to provide a beam spiller in which the rate and duration of beam spill may be accurately preset.

The invention, together with further objects and advantages thereof, will be best understood by reference to the accompanying drawing of which:

FIGURE 1 is a block diagram of the electronic Circuitry in the invention together with a plan view of a particle accelerator, and

FIGURES 2 and 3 are voltage vs. time curves of.potentials occurring in the accelerator under differing conditions.

Referring now to FIGURE 1, there is shown a particle accelerator 11, such as a high energy synchrotron, in which a closed beam orbit 12 is indicated, charged particles in a beam being deflected into such orbit by beam bending magnets 13, as described in detail in the above cited text Sourcebook on Atomic Energy on pages 265 to 268. In such accelerators, an accelerating electrode 14 is provided in a straight section 16 of the accelerator between magnet sections 13. It is a common practice to dispose a target 17 adjacent the beam orbit 12 at another straight section 18. After the beam particles are accelerated, the position of the beam orbit is altered slightly to cause all or a portion of the particles to strike the target 17, which is generally, but not necessarily, disposed on the inner side of the beam orbit.

A beam pickup electrode 19 is disposed at still another straight section 21 and is usually a hollow cylinder through which the beam particles pass, capacitively inducing a signal pulse in the electrode each time the charged particles pass therethrough around the beam orbit 12. Such signal has a waveform 22 in which (as with all the waveforms shown in FIGURE 1) the voltage amplitude is taken along the Y axis and time along the X axis. However,

waveform 22 is taken on a much shorter time scale than the other waveforms, which are all on the same time scale and for a time period approximately one million times longer than waveform 22. The pulses in waveform 22 are capacitively induced by the charged beam particles each time the beam passes through the electrode 19, the amplitude of the pulses indicating the total number of particles in the beam. A filter 23 averages the output signal from the pickup electrode 19 while a detector 24 provides a signal 26 which has a waveform indicating the quantity of total circulating beam current.

A sample and hold circuit 27 receives the output of the detector and, when suitably activated on by a start trigger signal applied at time t at a trigger signal input terminal 28, provides a steady-state output signal 29 corresponding to the amplitude of detector output waveform 26 at the moment when a trigger signal is received. The start trigger signal for the spill may be initiated in various ways depending upon the usage of the accelerator. Generally, a suitable start signal source is already available in the control circuitry associated with a synchrotron. In all the waveforms of FIGURE 1, the origin is assumed to occur at the time when a start signal is applied. Therefore, in detector output waveform 26, an initial amplitude A is indicated which corresponds to the steady-state amplitude of sample and hold output signal 29. Such signal 29 is inverted in inverter 31, thus providing a steady-state signal 32 with an amplitude of A. Prior to activation by a start trigger, and after receipt of a stop trigger signal at time 1 (discussed later) waveform 29 is identical to waveform 26. That is, when sample and hold circuit 27 is not holding, output signals therefrom are identical to applied input signals.

A first adder circuit 33 combines signals from two different sources, the output signal 32 from the inverter circuit 31 and a steady-state potential, the value of which is pre-selected by the setting of a potentiometer 34. Such potentiometer 34 is connected through switch 36 to a positive DC. power supply 37. The resultant output potential from first adder 33 is a steady-state potential which will have a value between zero and A. Such potential is inverted in inverter 38 and applied to the input of an integrator 39 through a potenitometer 40. The trigger on signal at terminal 38 also activates integrator circuit 39, which produces a linear ramp output signal 41, the slope of which depends upon the amplitude of the output potential from first adder 33 and the setting of potentiometer 40. A second adder circuit 42 receives three input signals, the detector output signal 26, first inverter signal 32, and integrator output signal 41. The three above signals are added together and the resultant is a negative feedback error signal 43 which has a small positive potential. Such positive potential signal 43 is applied to an amplitude control 44 for the RP. power supply 46 from which is supplied the accelerating potentials for the accelerating electrode 14 and causes the amplitude of the accelerating potential to decrease.

Generally, it is desirable to spill only a portion of the synchrotron beam at a time. Therefore, circuitry is provided so that a stop signal can be automatically generated by applying three input signals to a third adder 51, the three signals being the integrator output signal 41, the fraction of the potential 37 selected by potentiometer inverted in inverter 52. A stop signal is generated when a condition of zero voltage is detected at the output of adder 51 by a zero voltage detector and stop signal generator 53. The stop signal is applied to integrator 39 to cause such circuit to stop integrating and returns the output potential therefrom to a zero value. At the same time, the stop signal is applied to sample and hold circuit 27 and causes such circuit to cease holding an output signal corresponding to the input level thereto at the time the start trigger was applied.

Considering the operation of the invention, a mode of operation is considered where a predetermined quantity of current is to remain in the circulating beam after the initial spill. At first, a circulating beam is accelerated around the beam orbit 12 as in the standard operation of a synchrotron. Before a start trigger signal is applied, the detector output signal 26 and inverter output signal 32 are equal and opposite while the output of integrator circuit 39 is zero. Thus the output from adder circuit 42 is zero and the amplitude control 44 maintains the R.F. power supply level as shown in FIGURE 2 wherein the waveform 71 of a single cycle of the accelerator electrode 16 signal is shown. The beam particles arrive at the accelerator electrode and are accelerated during the phase interval generally indicated by bracket 72, in accordance with the usual synchrotron requirements for maintaining phase stability. When a start trigger is applied at terminal 28 to initiate the beam spill, the fixed output potential of sample and hold circuit 27 is applied (through inverter 31) to the adder circuit 33 where the amplitude is decreased by the fixed potential selected at potentiometer 34. Such resultant output signal from adder 33 is then integrated after polarity inversion by inverter 38, the slope of the output signal 41 being dependent upon the amplitude of the adder 33 output signal, and the setting of potentiometer 40. In adder 42, signal 26 and signal 32 are still equal and opposite at the start of the spill period, but integrator output signal 41 has risen from a zero value to a positive potential, thus the adder output signal has a slightly positive potential. The amplitude control responds to the positive signal by reducing the amplitude of the output potential from RF. power supply 46 thereby causing some of the beam particles to lose phase stability, as indicated in FIGURE 3. A sine wave accelerating electrode potential 76 of reduced amplitude is shown in FIGURE 3 where the beam particles now arrive during the bracketed interval 77. However, beam particles arriving prior to the peak 78 of the sine wave will lose phase stability and will quickly separate from the main portion of the beam and impinge on target 17. Upon the loss of such particles from the beam, the detector output potential 26 applied to adder 42 will become less positive and tend to decrease the positive potential at the ouput of adder 42, thus a negative feedback effect is provided. By such means, a controlled quantity of the beam is spilled at a constant rate, since, for example, if too many beam particles should spill at a given time, an immediate correction would be applied in that the detector output signal 26 would decrease in amplitude and reduce the amplitude of positive adder output signal 43, thus the amplitude of the RF. power supply output signal will increase and fewer beam particles would lose phase stability.

If desired, the spilling of the beam can continue at the fixed rate until all the beam is gone. However, it is frequently desirable to spill only a portion of the beam at a time. For instance, target 17 might be withdrawn and another target inserted to receive the remainder of the beam. Therefore, an automatic stop pulse is generated in zero voltage detector and stop signal generator 53 when the sum of the signals applied to adder 51 reaches a zero value. The output signal of the adder 51 will be negative until the integrator output signal 41 is sufficiently high to cause such output signal to reach a zero potential. A stop signal is generated and sample and hold circuit 27 is caused to return to the operating condition existing therein before the start signal was received. Thus the stop level is selected by varying potentiometer 34 while the rate at which particles are spilled is selected by potentiometer 40.

Another mode of operation is possible wherein a preselected constant portion of the total beam current is controllably spilled. Such a mode is useful since the quantity of current in the beam may vary considerably from pulse to pulse. To effect such mode, the switch 36 is operated to substitute the output potential of the sample and hold circuit for the power supply 37. It can be seen that the slope of the integrator output signal 41 will thus be automatically adjusted according to value of beam current at the time the sample and hold circuit 27 is triggered on.

The particular circuitry described could be altered in many ways and the particular signal polarities described could be changed. For instance, inverter 38 could be eliminated by utilizing a subtracting circuit in place of adder 33 and by reversing the polarity of power supply 37. Furthermore, a potential other than a ramp signal might be desirable to obtain a differing spill characteristic. That is, instead of the steady rate of spill provided by the ramp signal, a spill varying in intensity over the spill period is obtainable by utilizing a signal with differing waveshape. Thus, it will be obvious to those skilled in the art that numerous variations and modifications are possible within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims. I

What is claimed is:

1. In a spill control for a synchrotron in which charged beam particles are accelerated around a closed beam orbit by electrical waves applied at an accelerating electrode from a power supply, the combination comprising:

(a) means detecting the quantity of current in said beam,

(b) means deriving a spill control signal varying in amplitude,

(c) a negative feedback circuit combining an output signal from said detecting means and said spill control signal and providing a resultant amplitude control signal, and

(d) an amplitude control circuit for said power supply receiving said amplitude control signal from said negative feedback circuit, the amplitude of said electrical waves from said power supply being reduced correspondingly with increase in said amplitude control signal, whereby said beam particles are caused to leave said beam orbit at a controlled rate.

2. In a spill control apparatus for a synchrotron in which charged beam particles are accelerated around a closed beam orbit by electrical waves applied at an accelerating electrode from a radio-frequency power supply, the combination comprising:

(a) a beam pickup electrode disposed adjacent said beam orbit and providing an induced output signal having an amplitude dependent upon the quantity of particles in said beam,

(b) means producing a start trigger signal,

(c) a sample and hold circuit receiving said induced signal from said pickup electrode and upon activa tion by a start pulse from said trigger means producing a steady-state output signal having an amplitude corresponding to the amplitude of said induced signal at the time of occurrence of such start signal,

(d) a ramp signal generator receiving at least a portion of said steady-state output signal from said sample and hold circuit and having operation thereof triggered on by receipt of said start signal from said trigger means,

(e) an adder circuit receiving signals from the output of said ramp generator and said sample and hold circuit and said beam pickup electrode and providing a resultant control potential,

(f) means inverting signals applied to said adder from said sample and hold circuit with respect to signals from said ramp generator and said beam pickup electrode, and

(g) an amplitude control circuit for said radio-frequency supply and adapted to control the amplitude of said waves therefrom, said amplitude control reducing the amplitude of said waves upon receiving said control potential from said first adder circuit.

3. In apparatus as described in claim 2, the further combination of a filter connected to output of said beam pickup electrode, said filter being of the class averaging high frequency components in said induced signal, and an amplitude detector connected to the output of said filter and having an output connected to the inputs of said sample and hold circuit and said ramp generator.

4. In apparatus as described in claim 2, the further combination of a stop signal generator, stop pulses from said stop signal generator being coupled to said ramp generator and to said sample and hold circuit, said ramp generator being further characterized in that receipt of a stop signal thereat terminates the generation of a ramp signal, said sample and hold circuit being further characterized in that upon receipt of a stop signal thereat said sample and hold circuit ceases to hold a steady-state output potential.

5. Apparatus as described in claim 4 wherein a potential comparison means is provided in said stop signal generator, said comparison means receiving an output signal from said ramp generator and of the class generating an output stop signal upon receipt of a said signal from said ramp generator having an amplitude equal to a predetermined reference level.

6. Apparatus as described in claim 2 wherein said ramp generator is an integrator circuit.

7. In apparatus as described in claim 6, wherein said means inverting is a first signal inverter circuit connected to the output of said sample and hold circuit, and with the further combination of a fixed potential source, a potential level shifting means connected to said fixed potential source and said first signal inverter and shifting the value of output potentials from said first signal inverter by an amount equal to said fixed potential and a second inverter connected at the output of said level shifting means, said first inverter and said second inverter and said level shifting means being connected between said sample and hold circuit and said integrator circuit.

8. In apparatus as described in claim 6, wherein said means inverting is a first signal inverter connected to the output of said sample and hold circuit, and with the further combination of an adjustable voltage divider receiving output signals from said sample and hold circuit and having an output signal level corresponding to a selectable fraction of a signal level from said sample and hold circuit, a potential level shifting means connected to the output of said voltage divider and said first inverter and shifting the value of output potentials from said first inverter by an amount equal to output signals from said voltage divider, and a second inverter connected at the output of said level shifting means, said first inverter and said second inverter and said level shifting means being connected between said sample and hold circuit and said integrator circuit.

References Cited UNITED STATES PATENTS 3,005,954 10/1961 Heard 328235 X JAMES W. LAWRENCE, Primary Examiner.

R. IUDD, Assistant Examiner. 

